Durst built a thing called "LAVAKO". It was used to adapt the Agfa Varioscope color head (made in the '50s and '60s) to the L138. You had to remove the mirror, insert the LAVAKO (essentially just a tin box with sides cut 45deg) and mount the color head on top of the whole thing. The light from the mixing box in the color head went down in the top condensor, you could use all neg sizes when using a 6x9 mixing box (using different condensors, of course). The knob at the front was used to adjust the distance from light source to condensor to optimize lighting.
Originally Posted by Donald Miller
I adapted a 100W Meopta color head to it and used it for some time, replacing it with a real Durst color head eventually.